This invention relates to phased array antennas used in communication systems, and more particularly to an apparatus and method in which a phased array antenna is scanned or xe2x80x9choppedxe2x80x9d among several nodes of a TDMA network in a manner which allows closed loop pointing of the antenna relative to each node.
Forming and maintaining a communication link with an antenna, particularly a phased array antenna, having a narrow RF beam, is not a particularly easy task. For example, consider the problem of forming a communication link between two network nodes, node A and node B, where node A comprises a phased array antenna (PAA) and node B comprises a satellite dish based transponder, after they have agreed to form a communication link. Each node must further broadcast its location to all other nodes so that any node can compute the absolute direction to any other node.
The next step is determining which PAA covers the target node, in this example node B. For node A, this means estimating its own orientation in space and then using that orientation to compute the orientation of each PAA on the node, and finally assessing which PAA""s field of regard encompasses node B. The next step is to point the selected PAA""s beam at the target node. For the PAA which forms node A, this means computing the azimuth and elevation angles from the PAA boresight to node B. Finally, the signal is applied to the PAA of node A (for transmission) or to the demodulator thereof (for reception).
The steps described above constitute xe2x80x9copen-loop pointingxe2x80x9d, that is, pointing the beam in the known direction of the target without using feedback to improve pointing accuracy. If the PAA beam is quite wide, for example, twenty degrees or more, then open-loop pointing may provide a sufficient level of accuracy. If the beam is narrow, however, open-loop pointing is often not sufficient to achieve the needed degree of pointing accuracy. Sources of pointing error include misalignment of the antenna mount on the vehicle, for example an aircraft; twisting of the vehicles structure between the PAA and an attitude sensor of the vehicle; lag time between a vehicle maneuver (e.g., a roll or turn) and the response from the attitude sensor; atmospheric refraction of the beam; and ionospheric scintillation for Earth-to-space links.
To compensate for the errors described above, closed-loop pointing is frequently used. With closed-loop pointing operations, the received beam is not pointed steadily at the target. Rather, it is continually re-pointed (i.e., scanned) in various directions angularly offset from the target to scan for the direction of the strongest reception. This is illustrated in FIG. 1. FIG. 1 illustrates a five-point scan pattern. Each circle represents a fraction of the width of the beam from an antenna such as a PAA. In the first scan (circle number 1), the beam is pointed in the direction that is estimated to give the strongest reception. In the second through fifth scans, the beam is pointed at directions that are angularly offset from the direction of the first scan point. The angular offsets are determined such that each of the four quadrants about the periphery of the first scan point is covered by one scan point.
At each of the above illustrated scan points, a link control system operably associated with the antenna being pointed measures the received signal intensity at each scan point. After five scans, it uses the five measurements to compute a new estimate of the optimal pointing direction. In the next scan cycle, the pattern will center on the newly determined optimal pointing direction. This process of scanning the beam around a central point is sometimes called xe2x80x9cconingxe2x80x9d. Throughout the scan cycle, the link control system is not only measuring the signal strength but also passing the received signal to a demodulator associated with the antenna being pointed. That is, the link is transporting data while the controller maintains optimal pointing of the antenna.
The coning process described above is well known in the art. The specifics of how many points to use in the scan, how far apart they should be pointed (i.e., the angular offset), and how long each cycle should be are details decided to meet the needs of each application. Coning is well established for narrow-beam links where the beam tracks a single target for an extended period of time.
Until recently, narrow-beam antennas on mobile platforms, such as aircraft, could only be used for xe2x80x9ccontinuousxe2x80x9d links. By xe2x80x9ccontinuousxe2x80x9d it is meant those links where the RF beam points at a single target for an extended period of time. However, presently available PAA technology available from the Boeing Company allows a new type of RF link: the xe2x80x9cpoint-and-shootxe2x80x9d TDMA (Time Division Multiple Access) link. In this scheme, a single PAA beam is rapidly xe2x80x9choppedxe2x80x9d (i.e., scanned) among multiple targets (i.e. nodes). The advantage is that each platform gets most of the benefits of multiple high gain links (e.g. improved data rate, reduced interference and lower power consumption) without having to use multiple PAAs on the platform. This type of link is the enabling concept for several important projects of the Boeing Company which make use of a network incorporating of TDMA links involving PAAs to serve up to 200 or more mobile terminals.
When point-and-shoot TDMA links are implemented with small PAAs the beams are wide and there is little need for closed-loop pointing. However, for some platforms in planned military networks, large PAAs with narrow beams will be highly desirable. These high-gain PAAs would permit high data rates on communication links spanning many miles. Recent simulations conducted by the Boeing Company show that the use of TDMA links for a network making use of large PAAs can improve network throughput by 15-100% and reduce latency by about 25% compared to continuous links. For these long links, atmospheric refraction becomes a serious concern, so closed-loop pointing is required to maintain closure of each link.
In view of the foregoing, it would be highly desirable, then, to implement a closed-loop pointing system for use with an antenna operating as part of one node of a network having a large plurality of nodes, and which still can be used in a network requiring point-and-shoot TDMA links to allow the beam to be scanned (i.e. xe2x80x9choppedxe2x80x9d) rapidly between several nodes of the network.
The above and other objects are met by an apparatus and method for implementing closed-loop pointing of a beam of an antenna forming part of one node on a network having a large plurality of nodes, and where the beam is capable of being scanned from one node to another rapidly in accordance with a predetermined multiplexing sequence.
The method of the present invention involves assigning each node to specific slots of a repeating multiplexing sequence. In one preferred form, these slots form Time Division Multiple Access (TDMA) slots. For each node, several criteria are established. These criteria are: (1) an initial reported location for each node; (2) a desired number of scan points to be employed in determining the optimal pointing direction of an antenna of the system (representing part of one node) relative to a target node; (3) a center of scan (COS) field including azimuth and elevation information corresponding to the last-determined optimal pointing direction of the antenna relative to a given node; (4) a next scan point (NSP) field which defines a plurality of integers, wherein each integer is a representative of a unique pointing direction which is angularly offset from the last-determined optimal pointing direction for a given node, and wherein the plurality of scan points cooperatively represent a plurality of angularly offset pointing directions which circumscribe the last-determined COS information; and (5) a signal strength at last scan (SSLS) field for recording each one of a plurality of signal strength values determined for signals received by the antenna for a given one of the nodes, in correspondence with each one of the scan points, while the antenna is aimed at a given node.
The above information is used to allow the beam of the antenna to be rapidly scanned from one node to another in accordance with a TDMA multiplexing scheme, but which still enables closed-loop pointing of the beam at each node that the antenna has formed a communication link with during its scanning.
In performing the method of the present invention, the antenna is first aimed at a selected one of the nodes of the network in accordance with an initially reported location of the given node. The NSP value is then obtained. Information is then retrieved from the COS field which, at this point, represents an initially stored xe2x80x9coptimal pointing directionxe2x80x9d for the given node, and where this information allows the beam to be aimed at a center of the given node. Next, a unique translating function is used to generate azimuth and elevation coordinates for the retrieved scan point. The NSP field is then incremented and the antenna is pointed in accordance with the azimuth and elevation coordinates generated from the above-described unique translating function. The signal strength of a signal received from the target node is then recorded in a first sub-field of the SSLS field.
The above-described steps are repeated by retrieving the next scan point from the NSP field, again retrieving information stored in the COS field, again using a second unique translating function to generate azimuth and elevation coordinates for a second unique, angularly offset pointing direction corresponding to the second scan point, incrementing the NSP field, pointing the antenna in accordance with the azimuth and elevation coordinates generated from the second unique translating function, and recording the signal strength of the received signal. These steps are provided for each scan point until the last scan point in the NSP field has been used. The collective signal strength values stored in the SSLS field are then used in connection with a different, unique translating function to determine a new optimal pointing direction for the antenna which corresponds to the center of a given node. This new, optimal pointing direction is then stored in the COS field. When the above described steps are repeated, the new, optimal pointing direction information will be used from the COS field. In this manner, a new, optimal pointing direction will be determined each time all of the scan points in the NSP field have been used.
The above described operation is performed with respect to each node of the network that is targeted by the antenna. Since the present invention makes use of TDMA links, each node is assigned a time slot. The antenna (which is associated with one particular node) in one preferred implementation of the method, is directed at each node in accordance with that node""s timeslot. Thus, the method of the present invention allows a large number of nodes to be addressed by any other node, substantially simultaneously, such that the above-described closed-loop pointing operation can be performed by the antenna associated with each one of a large plurality of nodes. The ability of the specific types of antennas, particularly, PAA""s available from the Boeing Company, to scan from one target to another at rates of 10,000 times per second enables the method of the present invention to provide the needed closed-loop pointing accuracy substantially simultaneously to a large plurality of nodes.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.